The present invention relates to a liquid crystal dichroic display with cholesteric-homeotropic phase transition. The object is to achieve a better contrast of these displays used in reflection.
A liquid crystal cell comprises two walls separated by a shim between which is inserted a liquid crystal film, one wall carrying the transparent electrodes and the other a counterelectrode.
Dichroic displays are liquid crystal cells in which colouring agents or dyes are dissolved in the liquid crystal film. These dyes are elongated molecules, whose transition moment is generally directed along the molecule axis. They absorb the component of light, whose polarization vector is directed perpendicular to said axis, whilst excluding orthogonal components.
A first type of dichoic display is known, called a Heilmeier cell, which has a homeotropic-planar phase transition.
In the homeotropic phase, the molecules of the liquid crystal and dye are perpendicular to the cell walls. Thus, an incident light traversing the cell is transmitted without absorption.
In the planar phase, the molecules of liquid crystal and dye are aligned in a single direction contained in a plane parallel to the walls, so that they absorb the incident light. This absorption only takes place in one direction, which is that of the major axis of the dye molecules. To maximize the absorption, it is consequently necessary to provide a polarizer above the cell, so that the incident light is polarized in a linear manner parallel to the axis of the dye molecules before passing through the cell.
In the case where such a cell is used in reflection, the lower wall must be covered with a metallic coating, so that the reflected light retains its linear polarization, so that it is absorbed on return.
As a result of its absorption, the polarizer limits the brightness of the display. Moreover, the cost of this polarizer and its poor behaviour at high temperatures (approx. 90.degree. C.) and in the presence of moist atmospheres make this display relatively unsatisfactory.
Another type of dichroic display is known which requires no polarizer. This display, called a White-Taylor cell, uses a cholesteric-homeotropic phase transition.
In a cell of this type, use is made of a cholesteric liquid crystal, i.e. a liquid crystal whose molecules are chiral, and mixed with a dye. The assembly has a helical structure. The dye molecules are oriented in the direction of the liquid crystal molecules and consequently appear successively in all directions. Thus, the light passing through the cell is absorbed, no matter what its polarization.
This cell is preferably used in reflection because the light passing twice through the liquid crystal film is better absorbed, so that the contrast is improved.
It is also generally accepted that the base of the cell, on which the incident light is reflected, must be diffusing and must not preserve the polarization of the incident light. These diffusers are often outside the cell, and are e.g. made from paper or manganese oxide. The function of the diffuser is consequently to provide, following diffusion, with a random polarization to the reflected light.